1. Field of the invention
The present invention relates to a semiconductor device having a circuit constituted by thin film transistors (hereinafter, abbreviated as TFTs) and a method of manufacturing the same. For example, the present invention relates to an electro-optical device as is represented by a liquid crystal display panel and an electric appliance including such an electro-optical device as a component.
In the specification, the term “semiconductor device” generically indicates the devices capable of functioning by taking advantage of semiconductor properties; an electro-optical devices, a semiconductor circuit and an electric appliance are all included in the semiconductor device.
2. Description of the Related Art
A technique for constituting a TFT by using a semiconductor thin film (having a thickness of about several nm to about several hundreds of nm) formed on a substrate having an insulating surface has lately attracted attention. The TFT is widely used for electronic appliances such as an IC and an electro-optical device. Particularly, there is an urgent need to develop the TFT as a switching element for a liquid crystal display device.
Among liquid crystal display devices, an active matrix liquid crystal display device, in which pixel electrodes are arranged in a matrix and TFTs are used as switching elements respectively connected to pixel electrodes for obtaining a high-quality image, has particularly drawn attention.
The active matrix liquid crystal display device is mainly classified into two known types; a transmission-type and a reflection-type.
Particularly, as compared with the transmission-type liquid crystal display device, the reflection-type liquid crystal display device is advantageous in small power consumption because a backlight is not used for display. For such an advantage, the reflection-type liquid crystal display device is more and more demanded as a direct-view display for mobile computers or video cameras. The reflection-type liquid crystal display device makes use of the optical modulation effects of liquid crystal to select the state where incident light is reflected at the pixel electrode to be output to the outside of the device or the state where incident liquid is not output to the outside of the device so as to display a bright region or a dark region. The reflection-type liquid crystal display device further combines the bright region and the dark region to display an image. Generally, a pixel electrode in the reflection-type liquid crystal display device is made of a metal material having a high light reflectance such as aluminum, and is electrically connected to a switching element such as a TFT.
A pixel structure in a conventional reflection-type liquid crystal display device is shown in FIG. 23. In FIG. 23, two wirings, i.e., a gate wiring (scanning line) 11 and a capacitor wiring 12, are formed into a linear shape by patterning. A source wiring (signal line) 14 is also formed into a linear shape by patterning. The source wiring 14 is arranged in a row direction while the gate wiring 11 is arranged in a column direction. Between the source wiring 14 and the gate wiring 11, an interlayer insulating film is provided to insulate these wirings from each other. The source wiring 14 and the gate wiring 11 partially cross each other. This conventional structure is characteristic in that a TFT including a semiconductor layer 10 as an active layer is disposed in the vicinity of the intersection of these wirings.
Conventionally, it is known that a pixel electrode 15 is formed between the source wirings 14, simultaneously with the formation of the source wiring 14. In the case where such a structure is employed, a region between the source wiring 14 and the pixel electrode 15 is required to be shielded from light by using a light-shielding film (also referred to as a black matrix) 13.
Moreover, in the conventional structure, a metal film made of chromium or the like is patterned into a desired shape so as to obtain the light-shielding film 13. As a result of employing the conventional method, the number of steps and the number of masks for forming the light-shielding film itself increase. For sufficient light-shield with the light-shielding film 13, it is necessary to insulate the light-shielding film 13 and the pixel electrode from each other by providing an interlayer insulating film therebetween. Similarly with the light-shielding film, when the number of layers constituting the interlayer insulating film increases, the number of manufacturing steps correspondingly increases, resulting in a rise in manufacturing cost. Furthermore, this increase in the number of layers of the interlayer insulating film is disadvantageous in preservation of the interlayer insulation property.
As another conventional structure, a structure, in which an interlayer insulating film is provided so as to cover the source wiring 11 and the pixel electrode is provided thereon, is known. In the case where such a structure is used, however, the number of layers increases to increase the number of steps, resulting in a rise in manufacturing cost.
Furthermore, in terms of display performance, the pixel is desired to have a large amount of a storage capacitor as well as a high aperture ratio. A high aperture ratio of each pixel improves a light utilization efficiency to reduce the power consumption and the size of a display device.
Recently, the pixel size becomes smaller to obtain an image of higher definition. The reduction in pixel size increases the area where a TFT and a wiring are to be formed for one pixel, thereby adversely decreasing a pixel aperture ratio.
Thus, in order to obtain a high aperture ratio of each pixel with the defined pixel size, it is indispensable to effectively layout circuit components that are necessary for the circuit structure of a pixel.
As describe above, in order to realize a reflection-type liquid crystal display device or a transmission-type liquid crystal display device having a high pixel aperture ratio with the reduced number of masks, a completely novel pixel structure that does not correspond to any conventional structure is desired.
The present invention is to satisfy such a need and has a propose of providing a liquid crystal display device having a pixel structure of a high pixel aperture ratio realized by the present invention without increasing the number of masks and the number of manufacturing steps.